1. Field of the Invention
The present invention relates to an isocyanate-terminated prepolymer having an NCO content of from ≧26.0% by mass to ≦31.0% by mass, which is obtainable from the reaction of an isocyanate composition comprising from ≧30% by mass to ≦60% by mass dinuclear MDI and from ≧40% by mass to ≦70% by mass polymeric MDI with a bifunctional polyester ether polyol, which is obtained from the reaction of a dicarboxylic acid/dicarboxylic acid derivative with at least one polyol and with an epoxide. The invention further provides a PUR/PIR polymer prepared from this prepolymer, its use, and composite elements based thereon.
2. Description of Related Art
In the field of metal composite elements for the construction of factory buildings, high and permanent adhesion of the insulating foam to the metal cover layer is required. This is true in particular for polyisocyanurate foams. To that end there can be used as starting materials isocyanate-group-containing polyurethane prepolymers, which are obtained by reaction of a stoichiometric excess of an organic polyisocyanate with an organic polyol. Such prepolymers are well known in the field of polyurethanes and are used, for example, in the production of flexible and rigid foams and of elastomers, coatings, adhesive agents and the like.
The preparation of such NCO prepolymers has been described in many specifications. DE 69132613 T2, for example, discloses a process for the production of flexible foams which comprises the following steps:                continuously delivering an organic polyisocyanate composition and an isocyanate-reactive compound having an average molecular weight of at least 1000, and optionally a catalyst, to a reaction zone, the relative proportions of the isocyanate composition and of the isocyanate-reactive compound being suitable for the formation of a prepolymer having an NCO content in the range of from 2 to 15 wt. %;        reacting the isocyanate composition and the isocyanate-reactive compound in the reaction zone to form an isocyanate-terminated prepolymer;        continuously removing the reaction mixture from the reaction zone;        if necessary, allowing the reaction mixture to mature to form a prepolymer having an NCO content in the range of from 2 to 15 wt. %;        delivering the prepolymer to a foam-forming zone;        reacting the prepolymer in the foam-forming zone with an isocyanate-reactive component comprising water; and        recovering the foam so formed, the reaction zone also being used as the foam-forming zone.        
WO 94/29361 A1 describes a process for the production of flexible foam, in which a prepolymer having an NCO content of from 5 to 10 wt. % is used. The prepolymer is the reaction product of an excess of polyisocyanate having at least 85 wt. % 4,4′-diphenylmethane diisocyanate or a variant thereof with a polyol which has a nominal hydroxyl functionality of from 2.5 to 3.5, an average hydroxyl equivalent weight of from 1000 to 3000 and an oxyethylene content of from 50 to 85 wt. %. The prepolymer is reacted with water, the amount of water being at least 20 parts by weight per 100 parts by weight of the prepolymer.
DE 2823762 A1 relates to a process for the continuous preparation of thermoplastic polyurethanes from polyhydroxy compounds, diisocyanates and low molecular weight chain extenders, in which the starting components are passed through a mixing zone and then through a reaction zone. The starting components are mixed in a mixing zone in which virtually complete mixing sufficient for the subsequent polyaddition reaction is carried out. The product temperature is so low that a reaction of the components is largely avoided. The mixture is then fed by means of a precision pump into a static mixture of such a length and diameter that, at a flow rate that prevents the formation of polymers, the reaction components remain in the static mixer for a sufficient time that polyaddition takes place.
DE 19823392 A1 discloses a process for the preparation of isocyanate-group-containing prepolymers by reaction of organic isocyanates with H-acidic compounds, which process is characterised in that the reaction to the prepolymer takes place directly in the process stream of the isocyanate-producing plant. The publication additionally provides the isocyanate-group-containing prepolymers so prepared, their use in the production of PUR, and a process for the production of PUR by reaction of at least one isocyanate-group-containing prepolymer, optionally in admixture with further organic and/or modified organic isocyanates, with compounds having at least two reactive hydrogen atoms optionally in the presence of foaming agents, catalysts and optionally also further auxiliary substances and/or additives.
EP 0 497 131 A2 discloses isocyanate prepolymers containing ether and ester groups and having an NCO content of from 1.9 to 25 wt. % based on A) a polyisocyanate component consisting of at least one organic polyisocyanate having a mean molecular weight of from 168 to 1000, B) esterification products of polyether polyols having a mean hydroxyl number of from 200 to 600 with monocarboxylic acids, optionally in admixture with minor amounts of polybasic carboxylic acids, and optionally C) organic polyhydroxyl compounds of the molecular weight range from 62 to 200 in an amount of up to 25 hydroxyl equivalent %, based on component B), a process for the preparation of such isocyanate prepolymers by reaction of the mentioned starting materials while maintaining an NCO/OH equivalent ratio of from 1.7:1 to 20:1, and the use of the isocyanate prepolymers as binder or binder component in polyurethane coating compositions, sealing compositions or casting compositions.
DE 196 01 410 A1 relates to a process for the production of flexible polyurethane foams by reaction of a) organic and/or modified organic NCO-group-containing compounds with b) higher molecular weight compounds having at least two reactive hydrogen atoms and c) optionally low molecular weight chain extenders and/or crosslinkers in the presence of d) foaming agents, e) catalysts, f) flame retardants and optionally g) further auxiliary substances and/or additives. As organic and/or modified organic NCO-group-containing compounds there are used NCO-group-containing prepolymers which are reaction products of polyester polyols and/or polyester ether polyols and aromatic diisocyanates and which are subjected to a special after-treatment.
WO 2009/055436 A1 relates to polyester ether polyols and their use in urethane prepolymers, urethane foams and non-foamed urethane coatings, adhesives, sealing compositions and/or elastomers. Processes for the preparation of such polyester ether polyols with double metal cyanide catalysis and for the preparation of urethane prepolymers are described. The polyester ether polyols described therein are preferably the reaction product of phthalic anhydride, diethylene glycol and propylene oxide. The polyester ether polyols have improved solubility and compatibility in mixtures of either polyether polyols and/or polyester polyols. The polyester ether polyols are preferably less viscous than the polyester polyol intermediates and are generally soluble in either polyester and/or polyether polyols. In addition, the polyester ether polyols improve the hydrolytic stability of coatings, adhesives, sealing compositions and elastomers in which they are used.
The processes described in the prior art for the preparation of NCO prepolymers accordingly require a reaction of an isocyanate-group-containing component with an isocyanate-group-reactive component in a reactor and in particular under controlled mixing and temperature conditions.